Variable cut off attenuator for rectangular wave-guides

ABSTRACT

The attenuator includes two variable-section components ( 13, 15 ) for the passage, in conditions of perfect adaptation, from a first wave-guide in the P band to a second wave-guide in the X band and vice versa. An air-filled zone (5V), the length of which can be varied in function of the desired attenuation, is situated between the two variable-section components.

FIELD AND BACKGROUND OF THE INVENTION

The present invention refers to an attenuator for wave-guides and, morein particular, a so-called “cut-off” attenuator, i.e. a non-dissipativeattenuator with working frequencies below the cut-off frequency.

Rectangular wave-guides, that is to say those created in the form ofhollow components with a rigid rectangular cross-section along which themicrowaves propagate, are currently used in various applications. Tocreate cut-off attenuators for this type of wave-guide, cables placedbetween two portions of rectangular wave-guide are currently used. Thisis complicated from a constructional point of view. In particular, adouble guide-cable transition is required. For a description of thistechnique, refer to F. E. Terman “Electronic and Radio Engineering”,(McGraw Hill, New York, 1995, page 154).

In other forms of realization, the attenuator is composed of a platethat is inserted into the rectangular wave-guide. These attenuators alsoexhibit certain drawbacks known to the experts in this field.

SUMMARY OF THE INVENTION

The object of this invention is the realization of an attenuator forrectangular wave-guides and, in particular, a cut-off attenuator thateliminates the drawbacks of currently known attenuators.

In essence, the attenuator in accordance with the invention includes: afirst adapter with a first, variable-section component for the passage,in conditions of perfect adaptation, from a first wave-guide in a firstband to a second wave-guide in a second band, and a second adapter witha second variable-section component for the passage, in conditions ofperfect adaptation, from said second wave-guide to a third wave-guide insaid first band. Characteristically, the invention also prescribes thatthe first and second variable-section components are positioned in amanner such that they can slide within the second wave-guide and thatthe first and second variable-section components are mobile with respectto each other, allowing an empty part of longitudinally variable lengthto be defined between them in the second wave-guide.

The adapters for the passage from one wave-guide to another inconditions of perfect adaptation are described, for example, inIT-B-1253098 (application N° FI91A305), although the possibility ofusing these adapters in an attenuator is not mentioned.

In a practical form of embodiment, the variable-section components eachrespectively present an initial, pyramidal portion extending towardssaid first and said third wave-guides and a portion with a prismaticsection corresponding to the section of the second wave-guide, theprismatic portions terminating with their respective bases orthogonal tothe longitudinal axis of said second wave-guide.

Additional advantageous characteristics and forms of embodiment of theattenuator in accordance with the invention are indicated in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by referring to the descriptionand accompanying drawing, which illustrates a practical non limitingexample of said invention. In the drawing:

FIG. 1 illustrates an external view of the attenuator in accordance withthe invention,

FIGS. 2 and 3 illustrate a longitudinal section of the attenuator in twodifferent set-ups,

FIGS. 3A and 3B respectively illustrate a cross-sectional and a frontalview of one of the variable-section components along the lines IIIA—IIIAand IIIB—IIIB of FIG. 3,

FIG. 4 illustrates the theoretical attenuation curve,

FIG. 5 illustrates the real attenuation curve, and

FIG. 6 lists the experimental data acquired for constructing the curveFIG. 5.

DESCRIPTION OF TEE PREFERRED EMBODIMENTS

The structure of the attenuator in a possible form of embodiment isillustrated in detail in FIGS. 1 to 3. It presents two terminalconnectors, indicated as 1 and 3, joined together via a P-bandwave-guide indicated as 5, the ends of which are connected to the innerportions of the connections 1 and 3 via flanges 7 and 9 respectively.Frontally, the connectors 1 and 3 are associated with a first wave-guidein the X band, shown with a dot-dash line and indicated as 11, andanother wave-guide in the X band, shown with a dot-dash line andindicated as 12. As is known, wave-guides in the P and X bands have arectangular section with sides of different proportions. To connect eachX-band wave-guide 11 and 12 with the P-band wave-guide 5, the terminalconnectors 1 and 3 both have a respective cavity 1C and 3C, with avariable rectangular section that changes between the entrance and exitof the connector, i.e. between the guides 11 or 12 in the X band and theguide 5 in the P band.

A first component having a variable section, made of Teflon® forexample, extends inside connector 1 and has a pyramidal portion, thebase of which merges into a prismatic portion with a terminal base 13B(see detail in FIG. 3). The shape of the component 13 is also shown indetail in the sectional view in FIG. 3A and the frontal view in FIG. 3B.

A second component 15, with the same variable section extends inside theterminal connector 3, where it is symmetrically positioned with respectto component 13 such that its base 15B faces the base 13B of component13. The rectangular-section prismatic portions of the two,variable-section components 13 and 15 extend inside the wave-guide 5placed between the terminal connectors 1 and 3.

Each of the variable-section components 13 and 15 is connected via atongue, a key or some other connection member, schematically indicatedas 17 and 19, to a respective slider 21 and 23. The means of connection17 and 19 pass through a longitudinal slot 5F made in the wave-guide 5.The sliders 21 and 23 have threaded holes passing side-to-side thatengage with the threaded portions 25A and 25B, which are threaded inopposite directions, of a threaded rod 25 supported by brackets 27 and28 on the terminal connectors 1 and 3. The threaded rod 25 can bemanually rotated via a knob 31, solidly fixed to the rod via a shaft 33.Turning the knob 31 in one direction or the other results in thevariable-section components 13 and 15 sliding close together as shown inFIG. 2 or further apart, with the base surfaces 13B and 15B separatedfrom each other by an empty space 5V inside the rectangular guide 5. Thelongitudinal dimension of the empty space 5V can be adjusted by turningthe knob 31 to move the variable-section components 13 and 15 furtherapart or closer together.

Varying the longitudinal dimension of the empty space 5V, and hence thedistance between the variable-section components 13 and 15 gives rise toa variable attenuation of the impulse that is transmitted along thewave-guides 11, 5 and 12.

For a P-band guide (9.494 GHz cut-off frequency) and frequency range of8 to 9.49 GHz, the theoretical attenuation per unit length (cm) variesbetween 9.2 and 0 db respectively. This attenuation, expressed in db, isthe result of the formula (refer to F. E. Terman “Electronic and RadioEngineering”, McGraw Hill, New York, 1995, page 153): $\begin{matrix}{{\alpha\quad({db})} = {\left( {54.6/\lambda_{c}} \right)\sqrt{1 - \left( {v/v_{c}} \right)^{2}}}} & (1)\end{matrix}$where the cut-off frequency ν_(c) is expressed in GHz and the cut-offwavelength in cm is given by λ_(c)=30/ν_(c). FIG. 4 shows thistheoretical attenuation curve, with the frequency in GHz on thehorizontal axis and the attenuation per unit length on the verticalaxis. Theoretically, therefore, in the above described attenuator, anattenuation is obtained that depends on the propagated wave frequencyand which is equal to the ordinate value on the diagram in FIG. 4multiplied by the length of the empty space 5V, i.e. by the distancebetween the two surfaces 13B and 15B.

Nevertheless, Equation 1 is only valid if the coefficient oftransmission T is bound to the attenuation by the formula:T=exp(−α)  (2)which is only valid at the limit for large α values. More precisely, thecoefficient of transmission is given by:T=[1+exp(α)]⁻¹  (3)Thus, at the cut-off (ν=ν_(c)), the real attenuation α_(ν), which isbound to the coefficient of transmission by the relationα_(ν)(db)=10 log(T ⁻¹)  (4)is not zero but 3 db (since the coefficient of transmission is ½ and not1). Thus, the real attenuation curve is found to be that indicated bythe dashed line in FIG. 5, which also shows the measurements taken. Eachdata point, with relative error, is derived from a series of attenuationmeasurements in function of the length of the attenuating zone 5V for agiven frequency. FIG. 6 shows the results of these measurements for awave frequency of 8.59 GHz propagated through the wave-guides 11, 5 and12. The distance between the surfaces 13B and 15B is shown in mm on thehorizontal axis, while the attenuation in db is shown on the verticalaxis.

A multiplication factor of 0.8 was introduced to achieve a better fitbetween the theoretical curve (shown in FIG. 4) and experimental data(dashed line in FIG. 5), yielding the solid-line curve shown in FIG. 5.

It should be understood that the drawing merely illustrates an examplegiven as a practical demonstration of the invention, while the form andarrangement of said invention could be extensively changed withoutleaving the scope of the concept underlying the invention. The presenceof reference numbers in the attached claims is to facilitate reading theclaims in reference to the description and drawing and does not limitthe scope of protection represented by the claims.

1. An attenuator for rectangular wave-guides comprised of: a firstadapter with a first, variable-section component for the passage, inconditions of perfect adaptation, from a first wave-guide in a firstband to a second wave-guide in a second band, and a second adapter witha second variable-section component for the passage, in conditions ofperfect adaptation, from said second wave-guide to a third wave-guide insaid first band; wherein: said first and second variable-sectioncomponents are positioned so that they can slide within said secondwave-guide, said first and second variable-section components are mobilewith respect to each other to define between them, within said secondwave-guide, an empty zone with a variable longitudinal dimension, andsaid first and said second variable-section components each respectivelypresent an initial, pyramidal portion extending towards said first andsaid third waveguides and a portion with a prismatic sectioncorresponding to the section of the second wave-guide, the prismaticportions terminating with their respective bases orthogonal to thelongitudinal axis of said second wave-guide.
 2. An attenuator accordingto claim 1, wherein first and said second variable-section componentsare made of polytetrafluoroethylene.
 3. An attenuator according to claim1, wherein said first and said second variable-section components aremutually symmetrical.
 4. An attenuator according to claim 1, whereinsaid first and said wave-guides are X-band wave-guides and said secondwave-guide is a P-band wave-guide.
 5. An attenuator according to claim1, where said second wave-guide has a length of approximately 80 nm andthat the empty part has a variable longitudinal length, ranging from 0to 40 mm.
 6. An attenuator according to claim 1, wherein said first andsaid second variable-section components respectively extend into twocorresponding terminal connectors, between which said second wave-guideis inserted, the ends of which are connected to said terminalconnectors.
 7. An attenuator according to claim 6, wherein each of saidterminal connectors has a cavity with a variable rectangular sectionthat changes between the entrance and the exit of the connector.
 8. Anattenuator according to claim 6, where said second wave-guide presents alongitudinal slot through which pass members connecting said first andsecond variable-section components and corresponding sliders, in turnassociated with adjustment means for adjusting the relative position ofsaid first and second variable-section components.
 9. An attenuatoraccording to claim 8, wherein said adjustment means include a threadedrod with portions having threads in opposite directions and on whichsaid two sliders are engaged.
 10. A rectangular wave-guide attenuatorarrangement comprising: a first wave-guide in a first band; a secondwave-guide in a second band; a third wave-guide in said first band; afirst adapter with a first variable-section component defining a varyingsection space that changes between a wave entrance and a wave exit andwith wave propagation, in conditions of perfect adaptation, from saidfirst wave-guide in said first band to said second wave-guide in saidsecond band; a second adapter with a second variable-section componentdefining a varying section space that changes between a wave entranceand a wave exit and with wave propagation, in conditions of perfectadaptation, from said second wave-guide in said second band to saidthird wave-guide in said first band; support means for supporting saidfirst and second variable-section components so that they can slidewithin said second wave-guide, whereby said first and secondvariable-section components are mobile with respect to each other todefine between them, within said second wave-guide, an empty zone with avariable longitudinal dimension, wherein said first and said secondvariable-section components each respectively present an initial,pyramidal portion extending towards said first and said third waveguidesand a portion with a prismatic section corresponding to the section ofthe second wave-guide, the prismatic portions terminating with theirrespective bases ottbogonal to the longitudinal axis of said secondwave-guide.
 11. An attenuator for rectangular wave-guides comprised of:a first adapter with a first, variable-section component for thepassage, in conditions of perfect adaptation, from a first wave-guide ina first band to a second wave-guide in a second band, and a secondadapter with a second variable-section component for the passage, inconditions of perfect adaptation, from said second wave-guide to a thirdwave-guide in said first band; wherein: said first and secondvariable-section components are positioned so that they can slide withinsaid second wave-guide, said first and second variable-sectioncomponents are mobile with respect to each other to define between them,within said second wave-guide, an empty zone with a variablelongitudinal dimension, said second wave-guide presents a longitudinalslot through which pass members connecting said first and secondvariable-section components and corresponding sliders, in turnassociated with adjustment means for adjusting the relative position ofsaid first and second variable-section components, and said adjustmentmeans include a threaded rod with portions having threads in oppositedirections and on which said two sliders are engaged.